By using water to thin down Acrylic paints you will thin the paint, but at the expense of breaking down the Acrylic resins that bind the paint. The paint will be less resilient, and not bind as well to the surface.
You can thin the paint out to the point of separation; the pigments will start to split out with nothing you can do but throw it out and start again. The water thinned paint was also very frail once you had actually got it to cover the kit. When dry it was very prone to damage and rubbed off when handled!!
I started by thinning Acrylics with water and after a few disappointing results, tried a thinners and have never looked back.
I can't speak for or against
Tamiya thinners as I have never used them. I can however say that Vallejo's thinners work not only in Vallejo paints but also in Xtracrylix.
Ian M